The present invention is directed to cable TV systems wherein video signals are sent from a headend to individual settop terminals (STT) which are typically located in any convenient television viewing area. As is well known in the art, various levels of service are available to the set top terminals. Subscribers may choose from a wide variety of programs offered by a cable TV service. Since a cable TV service may provide many different programs, it is important to know which programs each individual set top is authorized to receive. Additionally, it is necessary to send other information from the headend to a set top for various other purposes as is well known in the art.
Prior systems used a separate data channel centered about 108.2 MHz to convey this information to the settop converters. The individual settop terminals each had a separate tuner which was always tuned to this frequency to receive any data that may have been sent down from the headend. Since this channel is outside the band of channels that are used for sending video signals, this type of system has commonly been referred to in the art as an "out-of-band" system.
One drawback to such a system is that each settop terminal needs a tuner to tune to the 108.2 MHz frequency as well as a variable tuner to tune to the selected channel. The requirement of two tuners in the settop significantly increases the cost of building these settops. There are other drawbacks to this system as is well known in the art.
For these and other reasons, it has become desirable to provide what is referred to as an "in-band" type system. Known in-band systems send authorization information and other information to the STT's over a channel that is used to transmit video information. One problem encountered with in-band type systems is that in order for a settop terminal to receive information from a headend or other source, it must be tuned to the frequency over which the in-band information is being transmitted. Therefore, if information is sent out from a headend to all the settop terminals, only those terminals that are tuned to that frequency will receive that information at that time. This gives rise to the need for providing refresh data, or put another way, it is necessary to repetitively send out these data messages to ensure that all settop terminals will receive them.
A problem further arises for cable system operators in that switching from an in-band system to an out-of-band system is costly and causes significant down time while converters are being switched. Moreover, prior systems required that each STT be replaced with an in-band set top terminal (ISTT) capable of receiving in-band formatted data from the headend.